This invention relates in general to apparatus for sensing movement, and in particular instances to devices for measuring strain in an object and devices for measuring acceleration.
Measurement of strain (the change in length of an object in some direction per unit undistorted length) in specimens and objects may be carried out either directly or indirectly. Some of the approaches used for direct strain measurements include the use of bonded wire strain gages (in which a grid of strain-sensitive wire is cemented to a specimen so that a change in the length of the grid due to strains in that specimen changes the resistance of the wire which can then be measured), mechanical strain gages (in which optical or mechanical lever systems are employed to multiply the strain which may then be read from a suitable scale), magnetic strain gages (which include magnetic circuits having air gaps which, when varied as a result of a strain in the specimens, varies the permeance of the circuits to provide an indication of the strains produced), semiconductor strain gages (in which the resistance of a piezoresistive material varies with applied stress and resulting strain in the material), and capacitance strain gages (in which a variation of capacitance caused by variation in the separation of elements due to strain in the specimen, can be measured to provide a reading of the strain). Other direct strain measuring devices include acoustic strain gages, brittle lacquer coatings, photo grids and cathetometers.
Approaches for indirectly measuring strain in a specimen include the use of displacement pickup devices, velocity pickup devices and acceleration detection devices.
A disadvantage of the conventional approaches to measuring strain (or forces including those produced by acceleration, weight, or the like), is that the devices employed are oftentimes difficult to attach to or to use with a specimen whose strain is to be measured. Also, such devices are typically difficult and costly to manufacture. Finally, because of the intrinsically high axial rigidity of many of such devices, it requires high quality bonding of the device to the specimen to prevent detachment due to failure of the bond and this, in turn, requires time-consuming and careful preparation of the specimen for bonding.
One approach to measuring forces in general has included the use of a parallelogram-type structure in which are mounted capacitive elements arranged to measure forces applied to the structures. Examples of some such devices include those disclosed in U.S. Pat. Nos. 4,092,856, 4,308,929, 4,572,006 and 4,649,759. All but one of the devices disclosed in these patents utilize variation of capacitance resulting from variation in the separation of the capacitive elements as the mechanism for measuring force. The capacitive elements are mounted on the structures in face-to-face relationships and so the structures are typically fairly bulky, having non-planar profiles. Such structures would be difficult to adapt for use as strain gages since, because of their bulk and size, it would be difficult to attach the structure to specimens or objects in which strain is to be measured. Also, fabricating the structures with the capacitive elements in the face-to-face relationship is quite difficult since multiple surfaces or faces of the structure must be prepared and treated.